Multi-channel optical transmitters provide a separate optical output for each of multiple optical communication channels. Multi-channel optical transmitters have been developed that include arrays of lasers, where each laser produces an individually modulatable light output. The light output from each laser may be modulated by direct modulation or external modulation. In a direct modulation approach, the output power of the laser typically is modulated directly by modulating the input drive current to the laser. In an external modulation approach, an external modulator modulates the intensity of light generated by a continuous wave laser in accordance with an information signal.
In one multi-channel optical transmitter approach, the modulated light outputs from the lasers are directly modulated and the modulated light outputs are coupled into respective optical fibers of an optical fiber array. Arrays of vertical cavity surface emitting lasers (VCSELs) and arrays of edge-emitting lasers, such as distributed feedback (DFB) lasers, have been used in these types of multi-channel optical transmitters. Currently, none of these types of multi-channel optical transmitters is capable of operating reliably at a data rate of 20 Giga-bits/second (Gb/s). For example, current VCSEL designs cannot reliably generate directly modulated optical signals at 20 Gb/s due to the high current densities that are required. Similarly, direct modulation of DFB lasers at 20 Gb/s is difficult to achieve. In addition, DFB lasers are extremely sensitive to back-reflections, which broaden the spectral linewidth and increase noise. For this reason, each of the DFB lasers in a multi-channel optical transmitter requires a separate optical isolator to protect the DFB lasers against back reflections, increasing the cost of such a multi-channel optical transmitter.
In another multi-channel optical transmitter approach, the light outputs from the lasers are directly modulated and the modulated light outputs are coupled into a single optical fiber. For example, a wavelength-division multiplexing (WDM) optical transmitter includes an array of lasers, such as DFB lasers, each of which is tuned to produce output light of a different respective wavelength. In order to transmit information over multiple communication channels, each laser in a WDM optical transmitter produces output light within a narrow wavelength range. The different output light wavelengths are combined in an optical coupler and the combined output light is injected into a single optical fiber.
In some multi-channel optical transmitters the light outputs from multiple DFB lasers are externally modulated by respective external electroabsorption modulators. The low chirp characteristics of the external modulators enable these types of multi-channel optical transmitters to achieve data rates of 20 Gb/s and higher. However, integrated arrays of DFB lasers and external electroabsorption modulators are difficult to manufacture and the power requirements of such multi-channel optical transmitters are significantly higher than other designs due to the need to drive both the laser array and the modulator array. In addition, in order to prevent reflections from feeding back into the DFB lasers, the output facet of each electroabsorption modulator typically has a high quality antireflection coating and a separate optical isolator is provided for each DFB laser. A direct active temperature-regulating device also typically is required to compensate for differences between the output wavelength temperature coefficient of the DFB lasers and the absorption edge wavelength temperature coefficient of the electroabsorption modulators. For this and other reasons, such externally modulated multi-channel optical transmitter designs tend to be bulky, expensive, and high in power consumption.